The present invention relates to a method of preventing cellular damage caused by electrophilic toxic, carcinogenic, or mutagenic agents.
It is well known that some chemical compounds are carcinogenic. Further, it is widely accepted that most chemical carcinogens either are strongly electrophilic or are metabolized by cellular enzymes to electrophilic derivatives. These electrophilic "ultimate" carcinogens react with nucleophilic groups in cellular macromolecules, and it is the macromolecular damage which leads to carcinogenesis, mutagenesis and toxicity. Classes of carcinogens which appear to act in this way include aromatic amines, the polycyclic aromatic hydrocarbons, nitrosamines and nitrosamides, and nitrogen mustards. In addition, some therapeutic toxicants, e.g. those used in cancer chemotherapy, may act in this way or may produce undesirable side effects through this mechanism. One strategy for intervention in these processes would be to provide a chemical scavenger of the toxic electrophiles, which would prevent macromolecular damage by prior reaction with the ultimate carcinogen.
This strategy has achieved limited success with two sets of compounds. The first set includes several phenolic plant compounds, exemplified by ellagic acid. These compounds scavenge electrophiles well in vitro, but have limited ability to block polycyclic aromatic hydrocarbon-included carcinogenesis in vivo. This is presumably due to problems with the pharmacokinetics of the compounds. A second set is the dithio-thiones. These were designed to be limited to direct acting, gut carcinogens since the chemopreventive compounds are not taken up by cells. This limits problems due to side effects, but also drastically restricts the range of carcinogens and target tissues to which the strategy is applicable.
In general, the limitations to the success of the nucleophilic scavenging strategy of chemoprevention are: 1) the production of undesirable side effects; 2) inability to achieve therapeutic concentrations intracellularly; and 3) inability to colocalize with the often hydrophobic carcinogens within subcellular compartments.
It is clear that the major xenobiotic metabolizing system of mammalian cells, the cytochrome P450 system, is microsomal in origin. For carcinogens such as the polycyclic aromatic hydrocarbons, which require metabolic activation, this is thought to lead to preferential localization of the metabolities in cellular membranes. Indeed, tests using a highly reactive, ultimate carcinogen derived from benzo(a)pyrene have shown that a pool of membrane-localized electrophile remains active in DNA binding for relatively long periods of time. Similar conclusions have been reached for the same electrophile in two-stage mouse skin carcinogenesis. Therefore, a potential chemopreventive agent should meet four criteria:
1. The agent should facilely detoxify electrophilic carcinogens, either by forming a covalent adduct or by catalysing other detoxification reactions or both;
2. The agent should be a substrate for an endogenous cellular transport system to ensure that it quickly reaches and maintains therapeutic intracellular concentrations;
3. The agent should have enough hydrophobic character to allow it to colocalize with those carcinogens which are hydrophobic; and
4. Toxicity should be minimal.
The compound 6-mercaptopurine (6-MP) has been found to satisfy the first three criteria. In addition, it is able to inhibit several biological activities (DNA damage, mutagenesis) of an ultimate carcinogen in Chinese hamster ovary cells. However, 6-MP is used as a cytotoxic agent in the treatment of childhood leukemia and therefore fails to meet the fourth criterion.
Therefore, a need exists for chemopreventive agents which can satisfy all these criteria.